Digital Radio Mondiale (DRM) is an open standard digital radio system for short-wave, medium-wave and long-wave communication. Audio source signals are typically encoded into digital signals and may be multiplexed with other digital data for transmission. The multiplexed audio signals and digital data may then be encoded by Quadrature Amplitude Modulation (QAM) to create Main Service Channel (MSC) cells. A Fast Access Channel (FAC) signal that contains information needed to find services and begin decoding the multiplexed signal may also be encoded by QAM to create FSC cells. Furthermore, a Service Description Channel (SDC) signal that provides information to decode services in the DRM transmission and to find alternate sources of data may also be encoded by QAM to create SDC cells.
The MSC, FAC and SDC cells may then be combined and an orthogonal frequency-division multiplexing (OFDM) signal generator used to create OFDM symbols representing the cells. The OFDM symbols may then be used to modulate a radio frequency signal for transmission to DRM receivers.
The DRM standard defines four ‘robustness modes’ of operation, intended to provide robust transmission under four types of signal propagation conditions. The transmitted DRM signal includes a succession of OFDM symbols, each symbol being made of a guard interval followed by a part of the symbol containing transmitted data. Each symbol is the sum of K sine wave portions (or carriers) equally spaced in frequency. Each sine wave portion, called a “cell”, is transmitted with a given amplitude and phase and corresponds to a carrier position. Each carrier is referenced by an index, or number.
The spacing between carrier frequencies and the number of carriers used to transmit a DRM signal are selected based upon a desired robustness mode of the signal and a desired frequency occupancy mode for the signal. The carriers are baseband signals and are used in a DRM transmitter to modulate a reference radio frequency signal.
A transmitted DRM signal is organized into transmission super frames. Each transmission super frame includes three transmission frames. A transmission frame includes a predetermined number of OFDM symbols, transmitted sequentially. The number of OFDM symbols is determined based upon a desired robustness mode, and is different for each mode. Under the DRM standard, a transmission frame may include pilot cells, control cells and data cells. The pilot cells may be used for frame, frequency and time synchronization, channel estimation and robustness mode identification. Pilot cells are selected cells modulated with predetermined phases and amplitudes.
Three frequency reference pilot cells are used by a DRM receiver to detect the presence of a received signal and estimate the signal's frequency offset. Frequency references are located at frequencies which are common to all variations of robustness mode and nominal channel bandwidth. Because the spacing between carrier frequencies varies, depending upon the robustness mode, the three frequency reference frequencies fall on three different carrier indices in each robustness mode. Frequency reference cells are transmitted in every symbol of every transmission frame with gain and phase of constant, known values.
Propagation conditions and lack of synchronization between transmitter oscillators and receiver oscillators may result in a frequency offset in a received DRM signal. The received signal may be offset from the expected reference RF frequency by an amount that is several times the carrier spacing of the transmitted signal. As such, the cells of a received DRM signal may be offset by several carrier indices. The number of carrier indices by which a received signal is offset is referred to as an integer carrier frequency offset.
Previous techniques for synchronizing a DRM receiver to a received signal having a frequency offset have been proposed. They typically require calculating several fast Fourier transforms (FFTs) on the received signal after its conversion from an RF signal to an IF signal. Such techniques will typically calculate multiple FFTs over a segment of received signal that includes several OFDM symbols. However, calculating an FFT is a computationally intensive operation, and calculating multiple FFTs is proportionally more intensive.
There is therefore a need for a technique for synchronizing a DRM receiver that reduces computational complexity. In particular, there is a need for a less computationally intensive technique for estimating a carrier frequency offset in a DRM receiver.